1. Field of Technology
The present invention relates generally to a drive apparatus for an optical disc recording and reproducing apparatus, and relates more particularly to technology for reducing the power consumption of the drive circuit for driving an actuator while assuring a high response rate in the drive circuit.
2. Description of Related Art
Optical disc media offer numerous benefits, including long media life as a result of non-contact recording and reading, random accessibility enabling significantly faster access to desired content than is possible with magnetic tape, and a large storage capacity. Disc drives for reading and/or writing Compact Disc (CD) and DVD (digital versatile disc) have thus become standard equipment on most personal computers sold today. In order to meet the demand for ever higher data transfer rates, faster disc motors are being used to rotationally drive the optical discs.
Optical disc drives used in desktop personal computers generally have a 5-V power supply with an output voltage of 5 volts, or a 12-V power supply with an output voltage of 12 volts. Two types of actuators are used in order to track the laser spot with an optical pickup, which is a type of movable head. One of these is a focusing actuator for adjusting the focus by moving the objective lens of the optical pickup in the focusing direction. The other is a tracking actuator for tracking the recording path by moving the objective lens in the tracking direction. The 5-V power supply and 12-V power supply are generally used to drive the focusing drive circuit and tracking drive circuit that respectively drive these actuators. The highest possible power supply voltage is used to drive the disc motor in order to increase disc speed, and a 12-V power supply is generally used in desktop personal computers. With increased integration of the DSP (digital signal processor) chips, a signal process circuit that operates with an approximately 3.3-V supply voltage is also increasingly incorporated in the optical disc drive.
Depending upon the optical disc drive operating conditions, the supply voltage to both the focusing and tracking drive circuits is preferably as high as possible. When the disc motor speed increases, for example, the servo also accelerates more quickly to the point being tracked on the disc. This requires operating both the focusing and tracking actuators at a higher rate of acceleration, and requires a stronger coil current. As a result, a high supply voltage is preferable for both the focusing and tracking drive circuits.
Increasing the access speed during optical disc playback requires moving and stopping the light spot at high speed in the tracking direction, and this requires operating the tracking actuator at a high rate of acceleration. This also makes a high supply voltage preferable for the tracking drive circuit.
A high supply voltage is also preferable for the focusing drive circuit when playing a disc with warp. When playing a disc with eccentricity, the supply voltage of the tracking drive circuit is preferably high. There are thus various circumstances in which a high power supply voltage is necessary.
The need for faster operation as described above is complicated by strong demand for low power consumption. The drivers for driving the actuators use PWM (pulse width modulation) control instead of linear drive control based on BTL drivers using bipolar transistors. This is because PWM drivers can reduce the power loss resulting from the internal voltage drop of the circuit. Noise output by the PWM driver as a result of high frequency current switching is a problem for the optical disc drive, however, and thus requires a separate unit for suppressing such circuit emissions.
More particularly, the playback signal output from the optical pickup can be extremely weak in an optical disc drive that can both read and write. If the PWM driver is used to drive the actuator that moves the objective lens of the optical pickup at this time, electrical noise from the PWM driver will interfere with the playback signal from the optical pickup. This can result in optical disc drive malfunctions and a higher error rate.
To avoid this problem, optical disc drives for both reading and writing quite commonly drive the relatively low frequency, high current consumption disc motor drive circuit with a PWM driver, and drive the relatively low current consumption, high frequency focusing and tracking drive circuits with a linear drive BTL driver.
The product of the current times the difference between the power supply voltage and the voltage actually applied to the actuator in the focusing and tracking drive circuits is lost as heat energy in the BTL driver circuit. When an optical disc is played at low speed, a high power supply voltage is not required for the drive circuit. However, because a high supply voltage (12V) is used for the power supply of the drive circuit, a significant amount of power is wasted and excess heat is generated in the linear drive type BTL drive circuits. In other words, if the power supply voltage supplying the drive current is high even though the current required to drive the focusing and tracking actuators is low, power consumption by the power output transistors increases, and heat and power consumption by the chips containing the linear drive type BTL driver become a problem.
Setting the power supply voltage no higher than is necessary is one way to avoid this problem. However, when something with greater deviation than is expected during normal operation occurs, such as when a disc with warping or eccentricity very near the specification limit is inserted to the optical disc drive, the drive current supplied to the actuator that moves the objective lens of the optical pickup becomes insufficient, and the ability of the optical disc drive to play such discs drops. There is thus a trade-off between disc playability and the ability to reduce heat output by lowering the supply voltage.
Recent hybrid driver chips combine the functions of the disc motor drive circuit and focusing and tracking drive circuits in a single device. When the disc motor is driven at high speed, the motor current to the disc motor increases, and power consumption by the disc motor drive circuit in the hybrid driver chip increases. If the high supply voltage (12V) is used as the power supply for the focusing and tracking drive circuits at this time, the internal power consumption of the hybrid drive chip increases and the chip temperature rises. If the optical disc drive is used in a high temperature environment, the temperature of the hybrid drive chip may even exceed the maximum temperature limit. The problems of power consumption and heat output are thus tending to become even more pronounced in IC devices having an on-board linear drive type BTL driver and optical disc drives that use such IC devices.
To address these problems and needs, Japanese Unexamined Patent Appl. Pub. 2003-132555 teaches technology for switching the power supply voltage of the focusing and tracking drive circuits according to the operating conditions of the optical disc drive. FIG. 1 shows the optical disc drive taught in Japanese Unexamined Patent Appl. Pub. 2003-132555. Note that only those parts needed to describe the operating principle of this optical disc drive are shown in FIG. 1.
Referring to FIG. 1, the optical pickup 130 emits a light beam to the optical disc 100, and the light reflected from the disc is converted to an electric signal that represents the information on the disc and is output to the playback signal processing circuit 140. The playback signal processing circuit 140 adjusts the amplitude of this playback signal, which is then demodulated by the playback signal demodulation circuit 150 to reproduce the information previously recorded on the optical disc 100.
The DSP unit 500 includes a microcomputer 510 and servo circuit 520. Rotation of the disc motor 110 is controlled by the disc motor drive circuit 120 based on signals output from the servo circuit 520 according to speed commands from the microcomputer 510, thereby driving the optical disc 100 at a specified speed.
The playback signal processing circuit 140 generates a focus error signal and a tracking error signal. The focus error signal indicates positioning error in the focal point of the laser beam in the focusing direction, and the tracking error signal indicates positioning error in the focal point of the laser beam in the tracking direction.
Based on the focus error signal output by the playback signal processing circuit 140, the servo circuit 520 controls the position of the focal point of the light spot in the focusing direction by the focusing drive circuit 220 and focus actuator 210 so that the light spot is focused on the recording surface of the optical disc 100. This is the focusing servo.
Based on the tracking error signal, the servo circuit 520 controls the position of the focal point of the light beam in the tracking direction by the tracking drive circuit 320 and tracking actuator 310 so that the light spot follows the recording track on the optical disc 100. This is the tracking servo.
In the optical disc drive taught in Japanese Unexamined Patent Appl. Pub. 2003-132555, the power switching circuit 400 switches appropriately according to the operating conditions of the disc drive between the 5V power supply 162 that outputs 5V, the 12V power supply 161 that outputs 12V, and the 3.3V power supply 163 that outputs 3.3V to supply power to the focusing drive circuit 220 and tracking drive circuit 320 and thereby reduce the power consumption of the optical disc drive.
This optical disc drive thus has a first power supply that supplies a first output voltage to the focusing drive circuit 220 and tracking drive circuit 320 during normal playback and recording conditions, and a second power supply that supplies a second output voltage that is higher than the first output voltage, and at least one of these power supplies is externally sourced. A switching unit switches to the second power supply when actuator drive power must be increased. A third power supply unit for stepping down the power supplied from the first or second power source to a third voltage that is lower than the first power supply voltage is also provided, and the power switching circuit 400 switches the power supply for the focusing drive circuit 220 or tracking drive circuit 320 to this third power supply based a command from the control unit.
The 3.3V power supply 163 that is the third power supply is often a DC-DC step-down converter, and a typical DC-DC step-down converter 23500 is described below with reference to FIG. 2.
This DC-DC step-down converter 23500 has a voltage comparator 23100, a PWM comparator 23200, a switching circuit 23300, and a step-down voltage generator 23400. The voltage comparator 23100 comprises a voltage amplifier 23110, resistance RC, and capacitance CC. The input terminals of the voltage comparator 23100 are the non-inverted input terminal and the inverted input terminal of the voltage amplifier 23110, and the output terminal of the voltage comparator 23100 is the output terminal of the voltage amplifier 23110. The output terminal of the voltage amplifier 23110 is connected to one side of the capacitance CC, the other side of the capacitance CC is connected to one side of the resistance RC, and the other side of the resistance RC is connected to the inverted input terminal of the voltage amplifier 23110.
The output terminal of the voltage comparator 23100 is connected to the inverted input terminal of the PWM comparator 23200, and the sawtooth wave 23210 voltage signal is input to the non-inverted input terminal of the PWM comparator 23200. The output terminal of the PWM comparator 23200 is connected to the control terminal controlling the switching circuit 23300. The switching circuit 23300 uses a pnp transistor 23310 and a regeneration current diode 23320. The emitter terminal of the pnp transistor 23310 is connected to the 5V power supply, and the collector is connected to the cathode terminal of the regeneration current diode 23320 and the input terminal of the next-stage step-down voltage generator 23400. The anode terminal of the regeneration current diode 23320 is to ground.
The step-down voltage generator 23400 comprises a coil L and a charging capacitor CS, and the input terminal is connected to one side of the coil L. One side of the charging capacitor CS is to ground. The other side of the coil L and the other side of the charging capacitor CS are connected to the step-down voltage output terminal of the step-down voltage generator 23400 from which the step-down voltage VC is output. The step-down voltage output terminal is connected to the inverted input terminal of the voltage amplifier 23110 in the voltage comparator 23100, and the 3.3V reference voltage is input to the reference voltage input terminal, which is the non-inverted input terminal of the voltage amplifier 23110 and the input terminal of the DC-DC step-down converter 23500.
Operation of the DC-DC step-down converter 23500 is described below.
The voltage amplifier 23110 compares the reference voltage of 3.3 V with the step-down voltage VC, and steps up the output voltage if the step-down voltage VC is lower than the 3.3V reference voltage to raise the voltage applied to the inverted input terminal of the downstream PWM comparator 23200.
The PWM comparator 23200 compares the voltage of the sawtooth wave 23210 input to the non-inverted input terminal with the inverted input terminal voltage, and outputs a PWM signal with a short pulse width if the inverted input terminal voltage rises. This PWM signal makes the ON period of the switching circuit 23300, which switches the 5V power supply voltage, longer than the OFF period, the step-down voltage generator 23400 thus converts the switched 5V supply voltage to a nearly direct current voltage, and the step-down voltage VC of the step-down voltage output terminal thus rises.
Conversely, if the step-down voltage VC is greater than the 3.3V reference voltage, the output voltage of the voltage amplifier 23110 drops and as a result the step-down voltage VC of the step-down voltage output terminal also drops.
As a result of this negative feedback operation, the DC-DC step-down converter 23500 operates to make the 3.3V reference voltage and the step-down voltage VC nearly equal.
Separately from the above, Japanese Patent 3513062 teaches technology for reducing collisions between the objective lens of the optical disc drive and the disc media. Optical disc drives that uses dual-layer optical disc media such as DVDs use a focus jump technique to control movement and focusing of the laser spot between recording layers in order to read data from two data recording layers from one side of the disc. Focus jump control may fail repeatedly at the same place on the disc as a result of localized scratches on the disc or accelerated disc speed due to disc warping. The technology taught in Japanese Patent 3513062 reduces collisions between the objective lens and disc by stopping jumping where jumping fails repeatedly.
The technology known from the literature as described above provides the power supply voltage of the focusing and tracking drive circuits of the optical disc drive from a first power supply used for normal playback, a second power supply that outputs a higher voltage than the first power supply, and a third power supply that outputs a voltage lower than the first power supply, and switches to the second power supply voltage when faster response is needed. The operating status of the optical disc drive is also analyzed in order to handle varying degrees of disc warping and eccentricity by appropriately switching the power supply voltage between the first, second, and third power supplies, thereby reducing power consumption. However, there is a limit to the degree that power consumption can be reduced using a control method that switches the supply voltage to the focusing and tracking drive circuits in three stages, and the conventional technology described above does not address applying the optimal supply voltage required by the drive circuits for low power consumption.
An object of the present invention is to solve this problem by further reducing power consumption by the drive circuits while also affording fast drive circuit response.